In the related art, a relay box disposed in an engine room or the like of a motor vehicle for distributing electric power from a vehicle-mounted power source to various types of electrical equipment is known (for example, see JP-A-2004-72907). In the relay box, a relay module including a housing accommodating a circuit component having a power circuit with switching devices or control circuits incorporated therein and a relay block including a synthetic resin casing accommodating plural plug-in relays may be mounted.